1. Field of the Invention
The present invention relates to an improved method for recovering hydrocarbons from a subterranean hydrocarbon-bearing formation containing low-gravity viscous oils or bitumens. More particularly, the invention relates to the recovery of viscous heavy oils from subterranean formations containing same by substantially reducing the soak time required in a miscible flood process.
2. Description of the Prior Art
There are many subterranean, petroleum-containing formations in various parts of the world from which substantial amounts of petroleum cannot be recovered because the viscosity of the petroleum is so great that it is essentially immobile at reservoir conditions. Thus, even if the formation contains adequate permeability, and an extraneous fluid such as water is introduced into the formation to drive the petroleum to a production well, little or no petroleum can be recovered from the formation because the viscosity of the petroleum is so great that it will not move. It is generally recognized that if the API gravity of the petroleum contained in the subterranean formation is less than about 25.degree., little or no recovery of the petroleum may be accomplished by conventional primary and secondary recovery means.
One of the most extreme examples of viscous petroleum containing formations from which essentially no production may be achieved by conventional primary or secondary means are the so called tar sands or bitumen sand deposits such as those located in the western United States, and the Athabasca Tar Sands in the northern portion of the province of Alberta, Canada, as well as in Venezuela. The viscosity of the bituminous petroleum contained in the Athabasca tar sand deposits, for example, is in the range of several million centipoise at the average formation conditions, thus, rendering the bituminous petroleum essentially immobile. Therefore, recovery of the bituminous petroleum from these deposits by any means other than mining has been essentially unsuccessful on any commercial level.
Some conventional thermal recovery methods have been applied to produce viscous hydrocarbons from formations and bitumens from tar sands among which are steam injection, hot water injection and in-situ combustion. Using these thermal methods,the viscous hydrocarbons are heated to temperatures at which their viscosity is sufficiently reduced and their mobility is sufficiently improved so as to enhance their flow through the pores of the formation.
Typically, such thermal techniques employ an injection well and a production well traversing the oil-bearing or tar sand formation. In a steam operation the heat furnished by the injected steam functions to lower the viscosity of the oil, thereby improving its mobility, while the fluid flow of the steam through the formation functions to drive the oil toward the production well from which the oil is produced. In the conventional in-situ combustion operation, characteristically much higher temperatures, i.e., above the ignition temperature of the crude, are obtained than in a steam operation.
Another technique that has been employed to recover viscous hydrocarbons is the use of hydrocarbon solvents. For example, it is well known that aromatic solvents, such as toluene and benzene, are capable of dissolving the heavier hydrocarbon components in heavy oils or bitumens, thereby improving their mobility by dilution. Aromatic solvents are generally more effective than parafinic-type solvents since the asphaltic components of the oils are more soluble in aromatic solvents. The solvents have a beneficial result in that they dilute the crude and thus make the crude more mobile due to the reduction in viscosity. However, their use has not been practical commercially since this process evolves long periods of soak-time to allow the solvent to mix with the crude. Therefore, the critical factor is the soak time needed, and depending on the thickness of the oil zone, the soak time may vary from a year or two up to possibly eight or more years.
It is thus an object of this invention to provide a recovery process wherein the soak time, required for solvent and crude oil mixing in a solvent flooding process, is significantly reduced. A substantial reduction in soak time is highly desirable as it would lead to a significant increase in oil production and allow earlier production, thus improving the process economics.